The Itoigawa-Shizuoka tectonic line (ISTL) active fault system is one of the longest and the most complex active fault systems in Japan. The system comprises the northern, the middle, and the southern segments. In the vicinity of the boundary of the middle and the southern segments (Hakushu - Nirasaki), tectonic landform related to the ISTL, active fault system is investigated and mapped. As a result of investigation, the following things became clear.
The Hakushu fault, with vertical slip rate of 0.2-0.4mm/yr, branches into two faults in a southernmost part. The eastern fault is reverse fault, and the western one is left lateral fault. Mukawa faults comprise many short faults with low activity (<0.1mm/yr). Mukawa faults are assumed to be a northernmost part of the Shimotsuburai fault. In the same location, broad flexure (Mukawa flexure), trending E-W, is formed. The Mukawa flexure uplifts southern side, with vertical displacement rate of 0.6mm/yr. It is estimated that the Mukawa faults accommodates a part of the displacement of the Mukawa flexure. Since branching end of the Hakushu fault and the Shimotsuburai fault are confronting each other, it is assumed that the segment boundary exists here.
The Hououzan fault has been considered to be an active fault formerly, based on the displacement of the terrace surface. As a result of detailed investigation, however, it become clear that the terrace was not deformed by any fault. Therefore, it is thought that the southern part of the Hououzan fault is not active recently. Based on the slip rate of faults, the displacement of the middle Pleistocene strata, and the estimation of subsurface structure, the history of fault activity is inferred as follows: (a) In late Neogene to early Quaternary, the Kamanashiyama - Hakushu - Hououzan faults were active. (b) In early to middle Quaternary, active segment jumped from the Hakushu - Hououzan faults to the Kamanashigawa - Shimotsuburai faults. These faulting uplifted the Akaishi Range, the Koma Mountains and the Nakayama. (c) In late Quaternary, the Kamanashigawa fault became inactive, and the Hakushu fault reactivated. The Mukawa faults (Mukawa flexure) were newly formed, and the active region of the fault system became the Kamanashiyama - Hakushu - Mukawa - Shimotsuburai faults. These faulting uplifted the Akaishi Range and Koma Mountains.